1. Field of the Invention
The present invention relates to an optical recording and reproducing apparatus and more particularly to an optical recording and reproducing apparatus suitably used when information is recorded on and reproduced from an optical recording medium having plural recording layers arranged in its layer direction.
2. Description of the Related Art
Nowadays, various optical recording media such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) are commercialized and widely spread. Among those recording media, in write-once media such as CD-R and DVD-R and rewritable media such as CD-RW and DVD-RW, a process for setting a laser power to an optimum value is performed first (OPC: Optimum Write Power Control), and a recording operation is then executed. For example, in the CD-R or DVD-R, as shown in JP 2002-260230 A for instance, power setting (OPC) is performed following β method. That is, test write is performed at a predetermined power on a power adjustment portion previously set on a disc to obtain a β value from its reproduction RF signal (asymmetry). Then, the obtained β value and a target β value (βt) desired on the disc are compared with each other to set the optimum power at the time of recording.
FIG. 10 shows a calculation method for the β value. As shown in FIG. 10, the β value is obtained by calculating β=(Itop+Ibtm)/(Itop−Ibtm) from asymmetry amplitude values Itop and Ibtm with respect to a reference potential Iref.
The laser power at the test write is set to an initial power previously recorded into a lead-in of the disc or an initial power previously set on the drive side as an initial value. Test write is performed using this initial power Pw1 to obtain the β value, the obtained β value and the target value βt are compared with each other to obtain a laser power Pw2 used at the next test write. Then, test write is performed again using the obtained power Pw2 to further obtain the β value.
When the test write is performed two times, as shown in FIG. 11, the test write is performed using the powers Pw1 and Pw2. The obtained β1 and β2 are subjected to linear approximation, and a laser power at which βt is given is obtained on the approximation line. The obtained laser power is directly set as the optimum laser power or verification is conducted whether the power can be set as the optimum laser power. In the verification, test write is performed using this power again, and an error rate when this test-written data is reproduced is obtained. Then, it is judged whether this error rate is smaller than a threshold, and when the error rate is smaller than the threshold, this laser power is set as the optimum laser power. After that, the recording operation starts at the thus set optimum laser power.
Incidentally, a disc having plural recording layers arranged on one side has been developed and commercialized recently. For example, JP 2003-346348 A discloses a DVD-R having two recording layers arranged on one side and a drive apparatus for the same.
When a disc has plural recording layers arranged on one side in this way, it is necessary to perform an optimum laser power setting process for each recording layer. However, in this case, if the power setting process is executed on each recording layer, there is a problem in that it takes a long time to complete the power setting process. For example, even when the number of times for performing the operation for obtaining the β value is limited to two as described above, if a checking is performed thereafter, it is necessary to perform the test write by three times and its reproducing process for each recording layer. Therefore, when there are arranged two recording layers, it is necessary to perform the test write by six times and the corresponding reproducing process.